The present invention relates to a data communications system in which it is desired to communicate a computer-generated stream of binary data signals from a transmitting site to a receiving site or a plurality of receiving sites. Generally, such systems involve a computer which controls a signal transmission system and a receiving system employing a computer for decoding the data and rendering it in useful form for performing a function. This invention had its origin in the development of automatic control systems relating to the control of consumer loads in an electrical utility distribution network. Such networks employ control systems for automatically controlling the environmental conditions within buildings and for turning on and off electrical power consuming appliances in order to obtain utility control over the distribution and use of electrical power in the network. While such use is specific, the data communications system developed for this purpose will be found to be widely applicable and to possess many unique features which may be employed singularly or collectively in a wide variety of data communications purposes.
In general, the direct control of electrical loads has been practiced for many decades using various technologies. Since the inception of a significant public awareness in the limitations of energy as a resource and because of significant cost increases, considerable effort has been made to develop direct, utility-operated load management operational systems. The principal proposed use requires a communication channel from the utility to the consumer's location. This has required an effective means for transmitting a data signal over the channel as well as a simple and reliable receiver capable of decoding the signal and employing the result to control load function, e.g., for turning loads on and off. Even this relatively simple objective is difficult to achieve.
The use of power lines of a power line distribution network as a communication channel is limited in many ways, but proposals have been made to do so. The power line systems suffer from a hostile noise environment and considerable interference appearing on the power lines themselves. In addition, propagation losses from radiation and from power transformer blockage is significant. Proposals to independently transmit control signals, on a VHF FM carrier for example, are met with limitations due to limited spectrum availability and limited power as well. Telephone communications involve not only costly installation and dedication of lines, but are also subject to considerable uncertainty particularly while a power outage condition is occurring and maximum utility control reliability is required. Telephone switching systems are often overloaded at these times and cannot function reliably. Proposals for the use of a simultaneous non-interfering modulation of existing AM and FM broadcast radio stations have also been made.
The present invention is a complete description of an operable system utilizing existing FM allocations and radio station power by broadcasting a control channel as an encoded modulation on the supplemental communication authorization (SCA) portion of the channel. The transmission is capable of co-sharing the SCA channel with typical users, such as background music systems, without interfering with or limiting such co-channel user and without interfering with the FM stereocast transmission.
The information which may be transmitted is so great that virtually any load function can be controlled. Specifically, the present invention discloses an apparatus for shutting the power to a load device off completely by interposing an operable relay system of high power transmission capability between the load and the associated electrical source such as at the outlet of the circuit breaker system.
It is common in systems of this type to refer to the transmission of data from the utility via a communications channel to the user as communication in the forward link. Forward link communication comprehends load control by e.g., switching devices on and off, causing mass shut-down of loads, or selective restarting of loads to bring them back on-line after a power-out condition has been corrected. The reverse link seeks to make available communication from the user to the utility. This is used for obtaining meter readings remotely and for other condition performance like feedback from the user. However, the enormous difficulty in providing every user with a private communication channel or a shared channel, as well as the high cost of such transmitters coupled with the virtual unavailability of radio channels for such private use, renders reverse link communication generally impractical.
Accordingly, the present invention proposes a data communications system of such capability and reliability that reverse link communication is believed unwarranted and is not proposed. Such communication would be possible if desired, however.
Of the considerable patent art which pertains to this subject, the following patents are given only as examples of the art. U.S. Pat. No. 4,117,405 to Louis Martinez, issued Sept. 26, 1978, is an example of a proposal for using available AM broadcast radio stations as a communications channel. It is proposed in Martinez that modulation be used by varying the phase while maintaining sychronization between the transmitter and receiver using phase-lock loop circuits. Also, polling of receivers is disclosed for avoiding difficulties in the amount of information that can be transmitted over a narrow band channel.
Also known is U.S. Pat. No. 3,980,954 to Ian A. Whyte, entitled BIDIRECTIONAL COMMUNICATION SYSTEM FOR ELECTRICAL POWER NETWORKS, issued Sept. 14, 1976, which proposes using a frequency shift keyed audio tone transmission on the SCA channel of commercial FM broadcast transmitters.
The AM broadcast proposes an FM/phase modulation of the carrier frequency. FCC regulations, however, dictate that the instanteous carrier frequency must be within .+-.20 Hz of the specified transmitter carrier frequency. This limits the peak frequency deviation to a few Hz such that the effective bandwidth is so narrow that the data rate is severely limited. The practical consequence of such limitations are that the capability of individual addressing on a large scale deployment will not be possible. For instance, to individually address even two million customers with a versatile compound structure and appropriate redundancy, a message length would have to be about 32 bits. It can be shown readily that in the AM case, even an efficient 1 bit per second per Hz encoding or 4 bit data rate per second would take about six months to address this number of customers. On the other hand, in an FM broadcast modulation approach, if the full SCA channel were utilized, where an available bandwidth might be 5,000 Hz, the required time to accomplish the same feat would be less than eight hours.
In addition to the foregoing, the AM systems proposed generally involve varying the phase or center frequency of the AM channel. Because this is proposed for AM stereo transmission, this data transmission approach appears to be incompatible with future AM stereo broadcast operations. In addition, AM transmission suffers from other difficulties relating to night time long distance propagation which causes interference from great distances over a valuable portion of the transmission time frame.
The proposal of U.S. Pat. No. 3,980,954 calls for transmission of a two tone FSK which would completely eliminate a co-channel SCA user, thus eliminating substantial transmission channel value. If an attempt is made to add a two tone signal in addition to SCA co-channel, significant intermodulation will occur in the left minus right stereo channel, i.e., at 53 Hz and below. In addition, any attempt to filter out the frequencies requires either that any co-channel use, background music for example, be reduced in bandwith to a point where it is unacceptable or that filters of extremely high quality be employed since the typical frequency proposed is so sensitive to the ear. The latter solution requires a retrofit of all existing user SCA receivers and is economically and practically unjustified.
There is, therefore, a need for a new and improved data communications system and particularly for such a system which is adaptable to a widespread co-channel use in commercial broadcasting for utility load control.